1. Field of the Invention
The present invention relates to electrostatic liquid spray systems, and in particular a nozzle for an electrostatic liquid spray system having an internal dielectric shroud.
2. Background of the Invention
An electrostatic spraying process charges either powder or liquid particulate to improve spray delivery and deposition. Advantages of the electrostatic charging are uniform spray cloud dispersion as well as improved uniformity and mass transfer efficiency in coating of target surfaces. In practice, many types of target surfaces are coated by electrostatic sprays; varying from agricultural crops to automobiles, appliances, furniture and many other manufactured goods. Unique opportunities for electrostatic spraying are still emerging. For example, recently developed applications involve coating of surfaces with sanitizing agents for odor control and the prevention of illness caused by virus and bacteria in areas of high human concentration such as hotels, hospitals, restaurants, schools, day care services, military installations and cruise ships.
In transport from an electrostatic nozzle, unipolar charged particles of relatively low mass maintain separation due to mutual repulsion and are driven along electric field lines to deposit uniformly. Sufficiently charged particulate clouds create strong space charge fields that propel particles near the edge of the spray cloud towards the target. The electrostatic forces due to this space charge are beneficially supplemented by image charge forces that aid the deposition process once individual particulates approach very close to the target substrate. These image charge forces are important to allow very small particles to overcome air boundary layer effects and deposit on the surface. A high ratio of particulate charge-to-mass is important to the process. Very small, highly charged particulates of high numerical density create beneficial space charge and image force fields, maximizing the electrostatic effects and minimizing the influence of gravity.
Choice of the optimal electrostatic charging method to employ for a particular application often depends on the type of spray compound and the target. For example, dry powder sprays to be delivered and deposited onto planar grounded surfaces may be suited for corona or triboelectric types of charging systems. Air assistance can be added to improve charged powder deposition for more complex three dimensional targets. Conductive liquids held in small containers and atomized by hydraulic or gas pressure may be suitably charged by direct contact of the liquid with high voltage probes. Insulating liquids and conductive liquids of relatively high resistivity can be atomized and charged reliably by electrohydrodynamic (EHD) methods as are known in the art. Conductive liquids, such as water based sprays of agricultural or sanitization chemicals, may present leakage current challenges in corona charging systems, EHD nozzles or high voltage contact systems, and may be more suited for charging by non-contact induction methods such as those disclosed in U.S. Pat. No. 3,698,635 to Sickles, U.S. Pat. No. 4,004,733 to Law, and U.S. Pat. No. 5,704,554 to Cooper and Law.
However, use of induction charging methods in the room, equipment and furniture sanitizing applications above typically requires field serviceability and robustness of the design to both servicing as well as providing continuous and extended use of the system. Therefore, it would be desirable to provide an electrostatic sprayer system that has a design robust enough for field servicing and provides the ability to operate continuously for extended periods of time with low electrical power requirement.